1. Field of the Invention
Embodiments of the present invention are directed to a papermaking clothing and associated system and method for producing a defined-width paper web and, more particularly, to a through-air drying papermaking system and associated clothing and method implementing a through-air drying fabric having laterally-spaced impermeable strips such that the fabric therebetween defines the width of a paper web.
2. Description of Related Art
In a representative papermaking process, a fibrous slurry (i.e., an aqueous wood pulp or cellulose fiber mixture) is deposited on a moving forming wire from a headbox. The open structure of the forming wire allows some of the water from the slurry to drain therethrough, wherein the remaining cellulose fibers adhere to each other to form a fibrous web. Since the forming wire moves in a machine direction during the deposition of the fibrous slurry, an elongate wet paper web is formed. Further, a representative papermaking machine as shown, for example, in FIG. 1, is often configured to produce a paper web of a certain width and, as such, the wet paper web formed on the forming wire must usually have the lateral boundaries trimmed in an edge trimming process. Edge trimming provides defined lateral edges of the paper web before the web is directed downstream in the machine direction for further processing, which may include, for example, pressing and/or drying sections.
In one edge trimming process, a high pressure water stream is directed through a water jet or nozzle toward the formed paper web as it is transported on the forming wire (i.e., the inner forming wire) in the machine direction, as shown, for example, in FIG. 2. The water is sprayed from the jet/nozzle to create a constant stream of water with high enough pressure to cut through, or in effect push aside fibers in a limited width of the sheet, but yet with low enough pressure and laminar flow to minimize spraying of water onto the rest of the paper web outside of the cut. The water flow must also be regulated to prevent damage to the forming wire. This edge trimming process is generally performed at between 12% and 30% dryness of the paper web, where the result is to define the outermost lateral edges of the paper web. In some papermaking processes, there may be a second cutting operation performed further downstream in the machine direction (i.e., later in the papermaking process), generally termed the inner edge cut. In any instance, the edge cutting or edge trimming system typically requires a significant amount of fresh water, piping and associated fixtures for controlling the flow of the water, various filters, a powered pump, and a spray jet/nozzle with an appropriate water control configuration for each cut and type of cut of the paper web. As such, the edge cutting or edge trimming system may be, for example, cost and maintenance intensive, resource (water) and energy inefficient, and difficult to correctly set up for alignment with, for instance, pickup vacuum box edges, molding box edges, and TAD-installed deckle bands along the machine direction in the papermaking process.
In some papermaking processes, once the paper web has proceeded through the edge trimming process, it is then directed through a dewatering process, such as a drying process. In one such drying process, one or more through-air dryers (TADs) may be implemented to dry the web. A typical TAD includes a cylindrical roll (otherwise referred to herein as a “TAD cylinder”), wherein the shell defining the cylindrical roll is configured and structured so as to allow air to pass through the cylindrical shell, about which the paper web is at least partially wrapped during the drying process. A TAD further includes a hood configured to substantially encompass the roll of the TAD, wherein air is typically heated and directed from the hood and into the roll through the shell, or from the roll through the shell and into the hood. In any instance, the air is directed through the paper web wrapped about the roll to facilitate drying thereof. The paper web, when transported through the TAD, is typically supported by an endless web-carrying fabric (otherwise referred to herein as a “TAD fabric”). Thus, the air directed through the paper web must also pass through the TAD fabric.
In some instances, however, the TAD fabric for transporting the paper web through the TAD may be a costly part of the paper production process. For example, mechanical deckle bands may be installed on the cylinder, in a laterally spaced apart relation, so as to define the “drying area” of the TAD. That is, such deckle bands may be, for example, impermeable strips of an impermeable material that are physically placed over the TAD cylinder at or about the edges/flanges thereof in order to block or re-direct air flow through the shell of the TAD cylinder. In such a configuration, the deckle bands are installed on the TAD cylinder at two spaced-apart positions across the width of the roll, and the TAD fabric is further configured to laterally extend across the roll and over both deckle bands. The width of the TAD fabric between the deckle bands thus defines the drying area of the TAD, where a paper web up to that width can be dried by the TAD. One disadvantage with such deckle bands, though, is that the placement thereof with respect to the roll for defining the drying area can be difficult to determine with accuracy due to, for example, the thermal expansion behavior of the roll. As such, temporary deckle bands may initially be used, with such temporary deckle bands being comprised of, for example, a polytetrafluoroethylene material, secured to the roll by temporary adhesives during set-up of the papermaking machine. This initial set-up, in some instances, may be costly in terms of the time and the trial-and-error resources needed to determine the appropriate positions of the deckle bands.
Once the appropriate positions of the temporary deckle bands are determined, deckle bands for use in the long term papermaking process can be installed on the roll. Such deckle bands may be comprised of a more durable material such as, for example, stainless steel, welded to the roll or the end rings thereof in the determined positions. However, one drawback of these metallic deckle bands is that, under certain conditions, the deckle bands may cause corrosion of the roll or the end rings thereof. Further, these deckle bands installed on the roll may be difficult to clean under/around. Also, between the initial set-up and actual (long term) production, machine parameters may be altered which may, in turn, change the requirements for the deckle bands. As such, the deckle bands may not be installed until immediately prior to production, which may result in delays and/or scheduling issues as a result of their implementation. As a result, the installation of the deckle bands for the long term papermaking process may also be costly in terms of time and resources.
In papermaking machines implementing a TAD having deckle bands affixed to the TAD roll, the paper web dried thereby is typically transferred to the TAD fabric or clothing such that there is an open lateral gap of the fabric between each edge of the paper web and the respective adjacent deckle band as shown, for example, in FIG. 3. Such a configuration may be necessary, for example, to minimize the risk of the paper web and/or the fabric/clothing shifting laterally such that the paper web extends outwardly of the deckle band, where it will not be dried. The heated air directed at the paper web supported by the fabric causes drying of the web through evaporation, and essentially protects the fabric from the full temperature of the heated air through, for example, an evaporative cooling mechanism. The paper web further causes resistance to the air passing therethrough, wherein the air is then more likely to flow through a path of least resistance which essentially comprises the fabric gap between each edge of the paper web and the respective deckle bands. However, as a result, the fabric gap is exposed to full temperature supply air during the web drying process, which heats the fabric only in the fabric gap area thereof. Generally, higher temperatures of the heated air minimize drying time for the paper web which, in turn allows the speed of the papermaking machine to be increased. However, the high temperatures of the drying air and/or mechanical wear at those higher temperatures may tend to cause the premature degradation of the fabric, particularly in the gap area. As such, frequent replacement of degraded fabric results in costs associated with the fabric replacement, as well as the costs of machine down time.
In order to address/minimize fabric degradation, some papermaking machines implement various fabric edge protection measures such as, for example, air knife edge cooling as shown, for example, in FIG. 4. In an air knife edge cooling process, cool air is blown onto the fabric gap area from immediately adjacent to the hot air supplied from a heated air supply duct for the TAD. The cool air is directed at the fabric gap, thereby creating a wall of cool air about the lateral edges of the paper web that minimizes the amount of hot air flowing though the fabric gap, while simultaneously cooling the fabric in the gap. However, air knife edge cooling may be sensitive to, for example, unbalanced air pressure (i.e., imbalance between the heated air and cooling air supply pressures) or the imprecise angular direction of the air supply. In such instances, a wet sheet or an ineffective air knife may result. Further, an air knife may be incapable of handling high temperature supply air found in some newer TAD papermaking machines. In addition, an air knife system may be equipment-intensive, requiring fans, ducting, sensors, and associated equipment. As such, the air knife system may be costly, complex and difficult to set up/install, difficult/expensive to alter for process changes, large, bulky, maintenance intensive, energy inefficient, and only marginally effective even when properly set up.
In some instances, a water spray edge protection system (see, e.g., U.S. Pat. No. 6,314,659) may also be implemented, as shown in FIG. 5, for protecting the fabric about the gap. Though this method is effective in protecting the fabric, much equipment may be required, correct setup thereof may be complicated, and major maintenance issues may be encountered.
Some existing devices and methods for addressing the fabric gap about each lateral edge of a paper web in a TAD papermaking machine thus may not provide a simple and effective method of changing the width of a paper web capable of being processed by the papermaking machine since the width of the paper web may often be determined by “permanently-installed” TAD deckle bands (or “deckles”). Further, efforts to address the fabric gap, as discussed above, may often be energy and resource inefficient (i.e., high energy consumption due to, for example, poor heat transfer and removal of water brought into the TAD by the fabric), and may overall be less than particularly effective for the intended purpose.
Thus, there exists a need for a system, apparatus and method for determining a width of a paper web in a papermaking machine, particularly a TAD papermaking machine in a process-effective manner. A solution should desirably involve minimal equipment, should be relatively simple and cost effective, should be capable of being readily altered for different web widths without extensive set up and testing requirements, and should facilitate maintenance of the papermaking machine. Such a solution should also desirably provide protection for the fabric gap of the drying fabric so as to prevent or minimize premature degradation thereof, while addressing energy consumption issues such as the amount of water brought into the TAD by the drying fabric, and a more complete and effective use of the heated air used in the TAD for drying the paper web.